Neural recordings in the brain using novel movable MEMS microelectrode arrays

Monitoring neural activity using microelectrodes holds tremendous opportunities to help create neural prosthetic devices. Microelectrodes have been used over the past 50 years, however, over the past couple of decades there has been a great demand to increase the longevity of these implants. Neural prosthetic devices often require the user to record from a group of neurons, however over time the neurons can migrate or move to a new location. Moveable microelectrodes that use microelectrical-mechanical systems (MEMS) actuator technology were used to solve these issues and increase the ability to maintain functionally stable cortical recordings. Moveable electrodes have been around for several years; however all of them use extremely large motors, which limit their use for chronic experiments. By using MEMS technology, the motors are able to reduce to micron size dimensions. However, the down side to using MEMS devices is that they need very reliable and durable packaging in order to operate. The MEMS devices are fabricated using the SUMMIT-V process at Sandia National Laboratory, and includes a 5-layer n-type doped polysilicon process. The actuators move the microelectrodes off the edge of the chip and into the brain, where they can be moved bi-directionally after implantation. This device is the first MEMS device with moveable components extending off the edge of the chip, which creates unique packaging challenges because traditional hermetic packaging is not feasible. This study validated the ability of highly doped n-type polysilicon microelectrodes as a chronic neural recording device. Signals were obtained from stationary microelectrodes for a period of up to 12 weeks with signal to noise ratios (SNR) of ∼15 dB. Corrosion effects under physiological conditions using various doping and fabricated polysilicon films were also studied. This study also shows a novel encapsulation technique that prevents fluid entry into a moveable microelectrode device. This study validates the ability to move the microelectrodes post implant in order to maintain functionally stable unit activity during semi-chronic recordings. The semi-chronic recordings show an average SNR of 14.61 +/- 5.2 and 18.13 +/- 4.99 dB before and alter movement respectively. This study also highlights the design and fabrication of a novel flexible chip-scale-package and bonding technique for implantable MEMS devices.